The present invention relates to superparamagnetic fine particles of iron oxide which are large in magnetization, excellent in oxidation stability and black in color; and the magnetic recording media containing the said particles which, owing to the presence of the superparamagnetic fine particles of iron oxide in the magnetic recording layer, are excellent in orientation property and packing property of the magnetic particles in the magnetic recording layer and also are minimized in necessary content of carbon black particles.
As one method for developing electrostatic latent images, there is known and popularly used a so-called single-component magnetic toner developing method using as developing agent the composite particles comprising magnetic particles such as magnetite particles mixed and dispersed in a resin without using a carrier.
With progressing longer recording time, and miniaturization size and weight reduction of both video and audio magnetic recording and reproducing devices, a request is arisen keenly for higher quality and performance, that is, higher-density recording and lower noise level of the magnetic recording media such as magnetic tapes and magnetic discs. For attaining this request, it is necessary that the magnetic particles used in the recording media are high in saturation magnetization, excellent in miscibility and also black in color.
This fact is pointed out, for instance, in Japanese Patent Application Laid-Open (KOKAI) No. 65406/80, which states: "Generally, the magnetic powder used for such single-component magnetic toner is required to have the following properties: i) The flux density in a magnetic field of about 10.sup.3 Oe is as high as it can be. For instance, it is necessary for the magnetic powder to have a maximum magnetization .sigma.m of about not less than 40 emu/g . . . iv) The magnetic particles have blackness that can stand practical use. A colorant may be contained in the magnetic toner, but it is desirable that the magnetic powder itself has a black color and no colorant is used . . . vii) The magnetic particles have good miscibility with resin . . . A microscopic degree of mixing in the toner is important for the properties of toner . . . "
Excellent dispersibility of the magnetic particles is essential for the improvement of miscibility of the magnetic particles with resin, and this requires that the magnetic attractive force of particles is small with minimized residual magnetization.
It is known that the blackness degree of magnetic particles such as magnetite particles is varied principally by average particle size and Fe.sup.2+ content, and it is required that the molar ratio of Fe.sup.2+ content to Fe.sup.3+ is not less than 0.16. Such fact is described in "Powder and Powder Metallurgy" Vol. 26, No. 7, pp. 239-240, 1979 (published by Japan Society of Powder and Powder Metallurgy) which states: "The blackness degree of the samples is variable depending on Fe(II) content and average particle size . . . When the Fe(II) content is more than 10%, all the samples have a black color although a slight difference is noted in the blackness degree. When the Fe(II) content decreases to less than 10%, each sample changes in color from black to reddish brown."
The magnetic particles which have hitherto been used for magnetic toner include (i) the octahedral magnetite particles produced by passing an oxygen-containing gas through a suspension containing ferrous hydroxide colloid and having a pH of not less than 10 obtained by reacting a ferrous salt solution with an alkaline solution of not less than one equivalent to Fe.sup.2+ in the said ferrous salt solution (Japanese Patent Publication No. 668/69), and (ii) the spherical magnetite particles obtained by, as the first step forming spherical particles by passing an oxygen-containing gas through a ferrous salt reaction solution containing ferrous hydroxide colloid obtained by reacting a ferrous salt solution with an alkali hydroxide of 0.80-0.99 equivalent to Fe.sup.2+ in the said ferrous salt solution, and after the said first step reaction, adding an alkali hydroxide of not less than 1.00 equivalent to residual Fe.sup.2+ and oxidizing the resultant mixture with a pH not less than 10 under heating (Japanese Patent Publication No. 51208/87).
On the other hand, with the request for higher image quality and performance of video tapes, the frequency of recorded carrier signal is increasing as compared with the conventional video tapes, that is, the signal frequency is shifting to a shorter wavelength region and, as a result, the depth of magnetization from the magnetic tape surface has been markedly lessened. Efforts are being made for further improving the high output characteristics while maintaining the desired level of CN ratio even for short wavelength signals. For attaining this, there are required 1 ultra-fine dividing of magnetic particles, 2 smoothening of magnetic coat surface, 3 reducing of thickness of magnetic coating film, and 4 high dispersing of magnetic particles.
This fact is referred to in, for instance, "Development of Magnetic Material and High-Degree Dispersion Techniques for Magnetic powder", p. 312 (1982) (published by Sogo Gijutsu Center) which states: "An essential condition for high-density recording on the coating-type tapes is to maintain a low noise level and high output characteristics for short wavelength signals. This necessitates a high coercive force (Hc) and a high residual magnetization (Br) as well as a small thickness of coating film", and an article in the May 3 issue of Nikkei Electronics (1976) on pages 82-105, which states: "The physical qualitative factors of tape associated with SN ratio (CN ratio) of luminance signal include average number of particles per unit volume, state of dispersion (dispersibility) of the particles and particle-surface smoothness. If the surface properties and dispersing properties are constant, the SN ratio is improved proportionally to the square root of the average number of particles, so that it is more advantageous to use a magnetic powder which can be made less in particle volume and higher in packing-degree."
As seen above, the common condition for satisfying the said requirements 2 to 4 for improving the high output characteristics is a high-dispersing of magnetic particles. In these days when there is a preference for use of ultra-fine particles whose high-dispersing is difficult, realization of high-dispersing of magnetic particles is of especially great significance.
On the other hand, stoppage of a magnetic tape, especially a magnetic recording medium such as video type, is conducted by detecting the portion with high light transmittance of the magnetic recording medium by a video deck. Detection by a video deck becomes difficult when light transmittance of the whole magnetic recording layer increases with thickness reduction of the magnetic recording medium or fine-division of magnetic particles as mentioned before, so that it is practiced to lower light transmittance by adding carbon to the magnetic recording layer. Thus, in the currently used video tapes, addition of carbon to the magnetic recording layer is essential.
The improvements of dispersibility of magnetic particles in a vehicle by use of the magnetic particles containing superparamagnetic iron oxide are proposed (Japanese Patent Publication Nos. 18766/83 and 12286/85).
The magnetic particles which are large in magnetization, small in magnetic attractive force owing to minimized residual magnetization and black in color are most acutely required at present. The octahedral magnetite particles (i) mentioned above have as high a value of magnetization as about 80 emu/g and are also excellent in blackness, but these particles have the problem in miscibility since they have residual magnetization of not less than 9 emu/g and tend to cause magnetic agglomeration. On the other hand, the spherical magnetite particles (ii) also mentioned above, although having as large a value of magnetization as over 80 emu/g, the particles have a brownish black color and are poor in blackness. Also, these particles have residual magnetization of about 4 to 6 emu/g, which is small in comparison with that of the octahedral magnetite particles but is still not satisfactorily small.
Superparamagnetic fine particles of iron oxide are known as the magnetic particles whose residual magnetization is almost zero. However, these superparamagnetic particles of iron oxide, because of extremely small particle size, are liable to be oxidized by oxygen in the air and very unstable chemically and magnetically. Thus, these particles, when exposed to the atmospheric air, undergo an oxidation reaction with oxygen in the air and are lowered in the magnetization. Further, the oxidation reaction of these particles proceeds during storage in the atmospheric air and their magnetization lowers in the passage of time. Also, the color of these particles changes to reddish brown due to oxidation of Fe.sup.2+ into Fe.sup.3+.
In view of the above, it is an object of the present invention to provide the superparamagnetic fine particles of iron oxide which have large magnetization and excellent resistance to oxidation.
On the other hand, it is most keenly required at present to improve dispersibility of magnetic particles in vehicle in the production of magnetic recording media. However, when carbon black particles are added in forming a coating composition by mixing a binder resin and magnetic particles, dispersion of magnetic particles in vehicle is disturbed, giving rise to the problem of deteriorated orientation property and packing property of the magnetic particles in the coating larger.
Further, carbon black particles are hard to handle and poor in workability since their apparent density is as low as about 0.1 g/cm.sup.3. Also, their problems in safety and hygienic aspects, such as carcinogenecity, are pointed out. On the other hand, use of the magnetic particles containing a superparamagnetic iron oxide such as disclosed in aforementioned Japanese Patent Publication Nos. 18766/83 and 12286/85 has the defect that the effect of reducing light transmittance is unsatisfactory although dispersibility of the particles in vehicle is improved.
It is a further object of the present invention to provide the magnetic recording media having very excellent dispersibility of magnetic particles in vehicle and a lowered light transmittance reducing a content of carbon black particles.
As a result of the present inventors' extensive studies for solving the said problems, it has been found that (1) by adding an unsaturated fatty acid or a salt thereof to an aqueous suspension containing superparamagnetic fine particles of iron oxide, stirring the mixture with a pH of not less than 9 at a temperature of 50.degree.-100.degree. C. to form a dispersion, adding an acid to the said dispersion so as to lower the pH to not more than 7, thereby adsorbing the unsaturated fatty acid on the surfaces of the said fine particles, then washing the thus obtained particles, dehydrating them, wetting them with an organic solvent miscible with water and finally drying them in a vacuum, the obtained superparamagnetic fine particles of iron oxide having an unsaturated fatty acid adsorbed on the particle surfaces, a magnetization of not less than 50 emu/g, the rate of change of the magnetization of not more than 10%, and the Fe.sup.2+ to Fe.sup.3+ molar ratio of 0.16-0.5, show an excellent blackness and are small in magnetic attractive force owing to zero residual magnetization and excellent in miscibility with resin; and (2) by coating on a substrate a coating composition comprising the said superparamagnetic fine particles of iron oxide, carbon black particles, magnetic particles and a binder resin, the total amount of the said superparamagnetic fine particles and carbon black particles being 0.1 to 10% by weight based on the magnetic particles and the weight ratio of the said superparamagnetic fine particles to the total amount of the said superparamagnetic fine particles and carbon black particles being 10 to 90, the obtained magnetic recording media are suited for use as magnetic recording media for high-density recording with low noise level owing to the excellent orientation property and packing property of the magnetic particles in the coating layer, and are also excellent in workability in the preparation of magnetic coating composition and advantageous from the aspects of safety and hygiene too, because of low content of carbon black. The present invention has been attained on the basis of these findings.